Isotopes of iodine

There are 37 known isotopes of iodine (53I) from 108I to 144I; all undergo radioactive decay except 127I, which is stable. Iodine is thus a monoisotopic element.

Its longest-lived radioactive isotope, 129I, has a half-life of 15.7 million years, which is far too short for it to exist as a primordial nuclide. Cosmogenic sources of 129I produce very tiny quantities of it that are too small to affect atomic weight measurements; iodine is thus also a mononuclidic element—one that is found in nature only as a single nuclide. Most 129I derived radioactivity on Earth is man-made, an unwanted long-lived byproduct of early nuclear tests and nuclear fission accidents.

All other iodine radioisotopes have half-lives less than 60 days, and four of these are used as tracers and therapeutic agents in medicine. These are 123I, 124I, 125I, and 131I. All industrial production of radioactive iodine isotopes involves these four useful radionuclides.

The isotope 135I has a half-life less than seven hours, which is too short to be used in biology. Unavoidable in situ production of this isotope is important in nuclear reactor control, as it decays to 135Xe, the most powerful known neutron absorber, and the nuclide responsible for the so-called iodine pit phenomenon.

In addition to commercial production, 131I (half-life 8 days) is one of the common radioactive fission-products of nuclear fission, and is thus produced inadvertently in very large amounts inside nuclear reactors. Due to its volatility, short half-life, and high abundance in fission products, 131I, (along with the short-lived iodine isotope 132I from the longer-lived 132Te with a half-life of 3 days) is responsible for the largest part of radioactive contamination during the first week after accidental environmental contamination from the radioactive waste from a nuclear power plant.

The portion of the total radiation activity (in air) contributed by each isotope versus time after the Chernobyl disaster, at the site. Note the prominence of radiation from I-131 and Te-132/I-132 for the first week. (Image using data from the OECD report, and the second edition of 'The radiochemical manual'.[2])
Main isotopes of iodine (53I)
Iso­tope Decay
abun­dance half-life (t1/2) mode pro­duct
123I syn 13 h ε, γ 123Te
124I syn 4.176 d ε 124Te
125I syn 59.40 d ε 125Te
127I 100% stable
129I trace 1.57×107 y β 129Xe
131I syn 8.02070 d β, γ 131Xe
135I syn 6.57 h β 135Xe
Standard atomic weight Ar, standard(I)
  • 126.90447(3)[1]
Pheochromocytoma Scan
A Pheochromocytoma is seen as a dark sphere in the center of the body (it is in the left adrenal gland). Image is by MIBG scintigraphy, with radiation from radioiodine in the MIBG. Two images are seen of the same patient from front and back. Note the dark image of the thyroid due to unwanted uptake of radioiodine from the medication by the thyroid gland in the neck. Accumulation at the sides of the head is from salivary gland uptake of iodide. Radioactivity is also seen in the bladder.

Notable radioisotopes

Radioisotopes of iodine are called radioactive iodine or radioiodine. Dozens exist, but about a half dozen are the most notable in applied sciences such as the life sciences and nuclear power, as detailed below. Mentions of radioiodine in health care contexts refer more often to iodine-131 than to other isotopes.

Iodine-129 as an extinct radionuclide

Excesses of stable 129Xe in meteorites have been shown to result from decay of "primordial" iodine-129 produced newly by the supernovas that created the dust and gas from which the solar system formed. This isotope has long decayed and is thus referred to as "extinct". Historically, 129I was the first extinct radionuclide to be identified as present in the early solar system. Its decay is the basis of the I-Xe iodine-xenon radiometric dating scheme, which covers the first 85 million years of solar system evolution.

Iodine-129 as a long-lived marker for nuclear fission contamination

Iodine-129 (129I; half-life 15.7 million years) is a product of cosmic ray spallation on various isotopes of xenon in the atmosphere, in cosmic ray muon interaction with tellurium-130, and also uranium and plutonium fission, both in subsurface rocks and nuclear reactors. Artificial nuclear processes, in particular nuclear fuel reprocessing and atmospheric nuclear weapons tests, have now swamped the natural signal for this isotope. Nevertheless, it now serves as a groundwater tracer as indicator of nuclear waste dispersion into the natural environment. In a similar fashion, 129I was used in rainwater studies to track fission products following the Chernobyl disaster.

In some ways, 129I is similar to 36Cl. It is a soluble halogen, fairly non-reactive, exists mainly as a non-sorbing anion, and is produced by cosmogenic, thermonuclear, and in-situ reactions. In hydrologic studies, 129I concentrations are usually reported as the ratio of 129I to total I (which is virtually all 127I). As is the case with 36Cl/Cl, 129I/I ratios in nature are quite small, 10−14 to 10−10 (peak thermonuclear 129I/I during the 1960s and 1970s reached about 10−7). 129I differs from 36Cl in that its half-life is longer (15.7 vs. 0.301 million years), it is highly biophilic, and occurs in multiple ionic forms (commonly, I and IO3), which have different chemical behaviors. This makes it fairly easy for 129I to enter the biosphere as it becomes incorporated into vegetation, soil, milk, animal tissue, etc.

Radioiodines I-123, I-124, I-125, and I-131 in medicine and biology

Of the many isotopes of iodine, only two are typically used in a medical setting: iodine-123 and iodine-131. Since 131I has both a beta and gamma decay mode, it can be used for radiotherapy or for imaging. 123I, which has no beta activity, is more suited for routine nuclear medicine imaging of the thyroid and other medical processes and less damaging internally to the patient. There are some situations in which iodine-124 and iodine-125 are used in medicine, also.[3]

Due to preferential uptake of iodine by the thyroid, radioiodine is extensively used in imaging of and, in the case of I-131, destroying dysfunctional thyroid tissues. Other types of tissue selectively take up certain iodine-131-containing tissue-targeting and killing radiopharmaceutical agents (such as MIBG). Iodine-125 is the only other iodine radioisotope used in radiation therapy, but only as an implanted capsule in brachytherapy, where the isotope never has a chance to be released for chemical interaction with the body's tissues.


Iodine-131 (131
) is a beta-emitting isotope with a half-life of eight days, and comparatively energetic (190 keV average and 606 keV maximum energy) beta radiation, which penetrates 0.6 to 2.0 mm from the site of uptake. This beta radiation can be used for the destruction of thyroid nodules or hyperfunctioning thyroid tissue and for elimination of remaining thyroid tissue after surgery for the treatment of Graves' disease. The purpose of this therapy, which was first explored by Dr. Saul Hertz in 1941,[4] is to destroy thyroid tissue that could not be removed surgically. In this procedure, 131I is administered either intravenously or orally following a diagnostic scan. This procedure may also be used, with higher doses of radio-iodine, to treat patients with thyroid cancer.

The 131I is taken up into thyroid tissue and concentrated there. The beta particles emitted by the radioisotope destroys the associated thyroid tissue with little damage to surrounding tissues (more than 2.0 mm from the tissues absorbing the iodine). Due to similar destruction, 131I is the iodine radioisotope used in other water-soluble iodine-labeled radiopharmaceuticals (such as MIBG) used therapeutically to destroy tissues.

The high energy beta radiation (up to 606 keV) from 131I causes it to be the most carcinogenic of the iodine isotopes. It is thought to cause the majority of excess thyroid cancers seen after nuclear fission contamination (such as bomb fallout or severe nuclear reactor accidents like the Chernobyl disaster) However, these epidemiological effects are seen primarily in children, and treatment of adults and children with therapeutic 131I, and epidemiology of adults exposed to low-dose 131I has not demonstrated carcinogenicity.[5]

Iodine-123 and iodine-125

The gamma-emitting isotopes iodine-123 (half-life 13 hours), and (less commonly) the longer-lived and less energetic iodine-125 (half-life 59 days) are used as nuclear imaging tracers to evaluate the anatomic and physiologic function of the thyroid. Abnormal results may be caused by disorders such as Graves' disease or Hashimoto's thyroiditis. Both isotopes decay by electron capture (EC) to the corresponding tellurium nuclides, but in neither case are these the metastable nuclides Te-123m and Te125m (which are of higher energy, and are not produced from radioiodine). Instead, the excited tellurium nuclides decay immediately (half-life too short to detect). Following EC, the excited Te-123 from I-123 emits a high-speed 127 keV internal conversion electron (not a beta ray) about 13% of the time, but this does little cellular damage due to the nuclide's short half-life and the relatively small fraction of such events. In the remainder of cases, a 159 keV gamma ray is emitted, which is well-suited for gamma imaging.

Excited Te-125 from EC decay of I-125 also emits a much lower-energy internal conversion electron (35.5 keV), which does relatively little damage due to its low energy, even though its emission is more common. The relatively low-energy gamma from I-125/Te-125 decay is poorly suited for imaging, but can still be seen, and this longer-lived isotope is necessary in tests that require several days of imaging, for example, fibrinogen scan imaging to detect blood clots.

Both I-123 and I-125 emit copious low energy Auger electrons after their decay, but these do not cause serious damage (double-stranded DNA breaks) in cells, unless the nuclide is incorporated into a medication that accumulates in the nucleus, or into DNA (this is never the case is clinical medicine, but it has been seen in experimental animal models).[6]

Iodine-125 is also commonly used by radiation oncologists in low dose rate brachytherapy in the treatment of cancer at sites other than the thyroid, especially in prostate cancer. When I-125 is used therapeutically, it is encapsulated in titanium seeds and implanted in the area of the tumor, where it remains. The low energy of the gamma spectrum in this case limits radiation damage to tissues far from the implanted capsule. Iodine-125, due to its suitable longer half-life and less penetrating gamma spectrum, is also often preferred for laboratory tests that rely on iodine as a tracer that is counted by a gamma counter, such as in radioimmunoassaying.

Most medical imaging with iodine is done with a standard gamma camera. However, the gamma rays from iodine-123 and iodine-131 can also be seen by single photon emission computed tomography (SPECT) imaging.


Iodine-124 is a proton-rich isotope of iodine with a half-life of 4.18 days. Its modes of decay are: 74.4% electron capture, 25.6% positron emission. 124I decays to 124Te. Iodine-124 can be made by numerous nuclear reactions via a cyclotron. The most common starting material used is 124Te.

Iodine-124 as the iodide salt can be used to directly image the thyroid using positron emission tomography (PET).[7] Iodine-124 can also be used as a PET radiotracer with a usefully longer half-life compared with fluorine-18[8] In this use, the nuclide is chemically bonded to a pharmaceutical to form a positron-emitting radiopharmaceutical, and injected into the body, where again it is imaged by PET scan.

Iodine-135 and nuclear reactor control

Iodine-135 is an isotope of iodine with a half-life of 6.6 hours. It is an important isotope from the viewpoint of nuclear reactor physics. It is produced in relatively large amounts as a fission product, and decays to xenon-135, which is a nuclear poison with a very large thermal neutron cross section, which is a cause of multiple complications in the control of nuclear reactors. The process of buildup of xenon-135 from an accumulated iodine-135 can temporarily preclude a shut-down reactor from restarting. This is known as xenon-poisoning or "falling into an iodine pit".

Iodine-128 and other isotopes

Iodine fission-produced isotopes not discussed above (iodine-128, iodine-130, iodine-132, and iodine-133) have a half lives of a couple of hours or minutes, rendering them almost useless in other applicable areas. Those mentioned are neutron-rich and so go through beta decay to their xenon counterparts. Iodine-128 (25 min half-life) can decay to either tellurium-128 by electron capture, or to xenon-128 by beta decay. It has a specific radioactivity of 2.177 x 106 TBq/g.

Non-radioactive iodide (I-127) as protection from unwanted radioiodine uptake by the thyroid

Colloquially, radioactive materials can be described as "hot," and non-radioactive materials can be described as "cold." There are instances in which cold iodide is administered to people in order to prevent the uptake of hot iodide by the thyroid gland. For example, blockade of thyroid iodine uptake with potassium iodide is used in nuclear medicine scintigraphy and therapy with some radioiodinated compounds that are not targeted to the thyroid, such as iobenguane (MIBG), which used to image or treat neural tissue tumors, or iodinated fibrinogen, which is used in fibrinogen scans to investigate clotting. These compounds contain iodine, but not in the iodide form. However, since they may be ultimately metabolized or break down to radioactive iodide, it is common to administer non-radioactive potassium iodide to insure that metabolites of these radiopharmaceuticals is not sequestered by thyroid gland and inadvertently administer a radiological dose to that tissue.

Potassium iodide has been distributed to populations exposed to nuclear fission accidents such as the Chernobyl disaster. The iodide solution SSKI, a saturated solution of potassium (K) iodide in water, has been used to block absorption of the radioiodine (it has no effect on other radioisotopes from fission). Tablets containing potassium iodide are now also manufactured and stocked in central disaster sites by some governments for this purpose. In theory, many harmful late-cancer effects of nuclear fallout might be prevented in this way, since an excess of thyroid cancers, presumably due to radioiodine uptake, is the only proven radioisotope contamination effect after a fission accident, or from contamination by fallout from an atomic bomb (prompt radiation from the bomb also causes other cancers, such as leukemias, directly). Taking large amounts of iodide saturates thyroid receptors and prevents uptake of most radioactive iodine-131 that may be present from fission product exposure (although it does not protect from other radioisotopes, nor from any other form of direct radiation). The protective effect of KI lasts approximately 24 hours, so must be dosed daily until a risk of significant exposure to radioiodines from fission products no longer exists.[9][10] Iodine-131 (the most common radioiodine contaminant in fallout) also decays relatively rapidly with a half-life of eight days, so that 99.95% of the original radioiodine has vanished after three months.

List of isotopes

Z(p) N(n)  
isotopic mass (u)
half-life decay
mode(s)[11][n 1]
isotope(s)[n 2]
spin and
(mole fraction)
range of natural
(mole fraction)
excitation energy
108I 53 55 107.94348(39)# 36(6) ms α (90%) 104Sb (1)#
β+ (9%) 108Te
p (1%) 107Te
109I 53 56 108.93815(11) 103(5) µs p (99.5%) 108Te (5/2+)
α (.5%) 105Sb
110I 53 57 109.93524(33)# 650(20) ms β+ (70.9%) 110Te 1+#
α (17%) 106Sb
β+, p (11%) 109Sb
β+, α (1.09%) 106Sn
111I 53 58 110.93028(32)# 2.5(2) s β+ (99.92%) 111Te (5/2+)#
α (.088%) 107Sb
112I 53 59 111.92797(23)# 3.42(11) s β+ (99.01%) 112Te
β+, p (.88%) 111Sb
β+, α (.104%) 108Sn
α (.0012%) 108Sb
113I 53 60 112.92364(6) 6.6(2) s β+ (100%) 113Te 5/2+#
α (3.3×10−7%) 109Sb
β+, α 109Sn
114I 53 61 113.92185(32)# 2.1(2) s β+ 114Te 1+
β+, p (rare) 113Sb
114mI 265.9(5) keV 6.2(5) s β+ (91%) 114Te (7)
IT (9%) 114I
115I 53 62 114.91805(3) 1.3(2) min β+ 115Te (5/2+)#
116I 53 63 115.91681(10) 2.91(15) s β+ 116Te 1+
116mI 400(50)# keV 3.27(16) µs (7−)
117I 53 64 116.91365(3) 2.22(4) min β+ 117Te (5/2)+
118I 53 65 117.913074(21) 13.7(5) min β+ 118Te 2−
118mI 190.1(10) keV 8.5(5) min β+ 118Te (7−)
IT (rare) 118I
119I 53 66 118.91007(3) 19.1(4) min β+ 119Te 5/2+
120I 53 67 119.910048(19) 81.6(2) min β+ 120Te 2−
120m1I 72.61(9) keV 228(15) ns (1+,2+,3+)
120m2I 320(15) keV 53(4) min β+ 120Te (7−)
121I 53 68 120.907367(11) 2.12(1) h β+ 121Te 5/2+
121mI 2376.9(4) keV 9.0(15) µs
122I 53 69 121.907589(6) 3.63(6) min β+ 122Te 1+
123I[n 3] 53 70 122.905589(4) 13.2235(19) h EC 123Te 5/2+
124I[n 3] 53 71 123.9062099(25) 4.1760(3) d β+ 124Te 2−
125I[n 3] 53 72 124.9046302(16) 59.400(10) d EC 125Te 5/2+
126I 53 73 125.905624(4) 12.93(5) d β+ (56.3%) 126Te 2−
β (43.7%) 126Xe
127I[n 4] 53 74 126.904473(4) Stable[n 5] 5/2+ 1.0000
128I 53 75 127.905809(4) 24.99(2) min β (93.1%) 128Xe 1+
β+ (6.9%) 128Te
128m1I 137.850(4) keV 845(20) ns 4−
128m2I 167.367(5) keV 175(15) ns (6)−
129I[n 4][n 6] 53 76 128.904988(3) 1.57(4)×107 y β 129Xe 7/2+ Trace[n 7]
130I 53 77 129.906674(3) 12.36(1) h β 130Xe 5+
130m1I 39.9525(13) keV 8.84(6) min IT (84%) 130I 2+
β (16%) 130Xe
130m2I 69.5865(7) keV 133(7) ns (6)−
130m3I 82.3960(19) keV 315(15) ns -
130m4I 85.1099(10) keV 254(4) ns (6)−
131I[n 4][n 3] 53 78 130.9061246(12) 8.02070(11) d β 131Xe 7/2+
132I 53 79 131.907997(6) 2.295(13) h β 132Xe 4+
132mI 104(12) keV 1.387(15) h IT (86%) 132I (8−)
β (14%) 132Xe
133I 53 80 132.907797(5) 20.8(1) h β 133Xe 7/2+
133m1I 1634.174(17) keV 9(2) s IT 133I (19/2−)
133m2I 1729.160(17) keV ~170 ns (15/2−)
134I 53 81 133.909744(9) 52.5(2) min β 134Xe (4)+
134mI 316.49(22) keV 3.52(4) min IT (97.7%) 134I (8)−
β (2.3%) 134Xe
135I[n 8] 53 82 134.910048(8) 6.57(2) h β 135Xe 7/2+
136I 53 83 135.91465(5) 83.4(10) s β 136Xe (1−)
136mI 650(120) keV 46.9(10) s β 136Xe (6−)
137I 53 84 136.917871(30) 24.13(12) s β (92.86%) 137Xe (7/2+)
β, n (7.14%) 136Xe
138I 53 85 137.92235(9) 6.23(3) s β (94.54%) 138Xe (2−)
β, n (5.46%) 137Xe
139I 53 86 138.92610(3) 2.282(10) s β (90%) 139Xe 7/2+#
β, n (10%) 138Xe
140I 53 87 139.93100(21)# 860(40) ms β (90.7%) 140Xe (3)(−#)
β, n (9.3%) 139Xe
141I 53 88 140.93503(21)# 430(20) ms β (78%) 141Xe 7/2+#
β, n (22%) 140Xe
142I 53 89 141.94018(43)# ~200 ms β (75%) 142Xe 2−#
β, n (25%) 141Xe
143I 53 90 142.94456(43)# 100# ms [> 300 ns] β 143Xe 7/2+#
144I 53 91 143.94999(54)# 50# ms [> 300 ns] β 144Xe 1−#
  1. ^ Abbreviations:
    EC: Electron capture
    IT: Isomeric transition
  2. ^ Bold for stable isotopes, bold italics for nearly-stable isotopes (half-life longer than the age of the universe)
  3. ^ a b c d Has medical uses
  4. ^ a b c Fission product
  5. ^ Theoretically capable of spontaneous fission
  6. ^ Can be used to date certain early events in Solar System history and some use for dating groundwater
  7. ^ Cosmogenic nuclide, also found as nuclear contamination
  8. ^ Produced as a decay product of 135Te in nuclear reactors, in turn decays to 135Xe, which, if allowed to build up, can shut down reactors due to the iodine pit phenomenon


  • Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
  • Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC, which use expanded uncertainties.


  • Isotope masses from:
    • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
  • Isotopic compositions and standard atomic masses from:
  • Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.
  1. ^ Meija, Juris; et al. (2016). "Atomic weights of the elements 2013 (IUPAC Technical Report)". Pure and Applied Chemistry. 88 (3): 265–91. doi:10.1515/pac-2015-0305.
  2. ^ "Nuclear Data Evaluation Lab". Archived from the original on 2007-01-21. Retrieved 2009-05-13.
  3. ^ Augustine George; James T Lane; Arlen D Meyers (January 17, 2013). "Radioactive Iodine Uptake Testing". Medscape.
  4. ^ Hertz, Barbara; Schuleller, Kristin (2010). "Saul Hertz, MD (1905 - 1950) A Pioneer in the Use of Radioactive Iodine". Endocrine Practice. 16 (4): 713–715. doi:10.4158/EP10065.CO. PMID 20350908.
  5. ^ Robbins, Jacob; Schneider, Arthur B. (2000). "Thyroid cancer following exposure to radioactive iodine". Reviews in Endocrine and Metabolic Disorders. 1 (3): 197–203. doi:10.1023/A:1010031115233. ISSN 1389-9155. PMID 11705004.
  6. ^ V. R. Narra; et al. (1992). "Radiotoxicity of Some Iodine-123, Iodine-125, and Iodine-131-Labeled Compounds in Mouse Testes: Implications for Radiopharmaceutical Design" (PDF). Journal of Nuclear Medicine. 33 (12): 2196.
  7. ^ E. Rault; et al. (2007). "Comparison of Image Quality of Different Iodine Isotopes (I-123, I-124, and I-131)". Cancer Biotherapy & Radiopharmaceuticals. 22 (3): 423–430. doi:10.1089/cbr.2006.323. PMID 17651050.
  8. ^ BV Cyclotron VU, Amsterdam, 2016, Information on Iodine-124 for PET
  9. ^ "Frequently Asked Questions on Potassium Iodide". Food and Drug Administration. Retrieved 2009-06-06.
  10. ^ "Potassium Iodide as a Thyroid Blocking Agent in Radiation Emergencies". Federal Register. Food and Drug Administration. Archived from the original on 2011-10-02. Retrieved 2009-06-06.
  11. ^ "Universal Nuclide Chart". nucleonica. (Registration required (help)). Cite uses deprecated parameter |registration= (help)

External links


Amiodarone is an antiarrhythmic medication used to treat and prevent a number of types of irregular heartbeats. This includes ventricular tachycardia (VT), ventricular fibrillation (VF), and wide complex tachycardia, as well as atrial fibrillation and paroxysmal supraventricular tachycardia. Evidence in cardiac arrest, however, is poor. It can be given by mouth, intravenously, or intraosseously. When used by mouth, it can take a few weeks for effects to begin.Common side effects include feeling tired, tremor, nausea, and constipation. As amiodarone can have serious side effects, it is mainly recommended only for significant ventricular arrhythmias. Serious side effects include lung toxicity such as interstitial pneumonitis, liver problems, heart arrhythmias, vision problems, thyroid problems, and death. If taken during pregnancy or breastfeeding it can cause problems in the baby. It is a class III antiarrhythmic medication. It works partly by increasing the time before a heart cell can contract again.Amiodarone was first made in 1961 and came into medical use in 1962 for chest pain believed to be related to the heart. It was pulled from the market in 1967 due to side effects. In 1974 it was found to be useful for arrhythmias and reintroduced. It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system. It is available as a generic medication. In the developing world the wholesale cost as of 2014 is about US$0.06–0.26 per day. In the United States a typical month supply is between $100 and $200. In 2016 it was the 198th most prescribed medication in the United States with more than 2 million prescriptions.


Caesium (IUPAC spelling) or cesium (American spelling) is a chemical element with symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of 28.5 °C (83.3 °F), which makes it one of only five elemental metals that are liquid at or near room temperature. Caesium has physical and chemical properties similar to those of rubidium and potassium. The most reactive of all metals, it is pyrophoric and reacts with water even at −116 °C (−177 °F). It is the least electronegative element, with a value of 0.79 on the Pauling scale. It has only one stable isotope, caesium-133. Caesium is mined mostly from pollucite, while the radioisotopes, especially caesium-137, a fission product, are extracted from waste produced by nuclear reactors.

The German chemist Robert Bunsen and physicist Gustav Kirchhoff discovered caesium in 1860 by the newly developed method of flame spectroscopy. The first small-scale applications for caesium were as a "getter" in vacuum tubes and in photoelectric cells. In 1967, acting on Einstein's proof that the speed of light is the most constant dimension in the universe, the International System of Units used two specific wave counts from an emission spectrum of caesium-133 to co-define the second and the metre. Since then, caesium has been widely used in highly accurate atomic clocks.

Since the 1990s, the largest application of the element has been as caesium formate for drilling fluids, but it has a range of applications in the production of electricity, in electronics, and in chemistry. The radioactive isotope caesium-137 has a half-life of about 30 years and is used in medical applications, industrial gauges, and hydrology. Nonradioactive caesium compounds are only mildly toxic, but the pure metal's tendency to react explosively with water means that caesium is considered a hazardous material, and the radioisotopes present a significant health and ecological hazard in the environment.

Fission products (by element)

On this page, a discussion of each of the main elements in the fission product mixture from the nuclear fission of an actinide such as uranium or plutonium is set out by element.


The halogens () are a group in the periodic table consisting of five chemically related elements: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At). The artificially created element 117 (Tennessine, Ts) may also be a halogen. In the modern IUPAC nomenclature, this group is known as group 17. The symbol X is often used generically to refer to any halogen.

The name "halogen" means "salt-producing". When halogens react with metals they produce a wide range of salts, including calcium fluoride, sodium chloride (common table salt), silver bromide and potassium iodide.

The group of halogens is the only periodic table group that contains elements in three of the main states of matter at standard temperature and pressure. All of the halogens form acids when bonded to hydrogen. Most halogens are typically produced from minerals or salts. The middle halogens, that is chlorine, bromine and iodine, are often used as disinfectants. Organobromides are the most important class of flame retardants. Elemental halogens are dangerous and can be lethally toxic.


Iodine is a chemical element with symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a lustrous, purple-black non-metallic solid at standard conditions that melts to form a deep violet liquid at 114 degrees Celsius, and boils to a violet gas at 184 degrees Celsius. The element was discovered by the French chemist Bernard Courtois in 1811. It was named two years later by Joseph Louis Gay-Lussac from this property, after the Greek ἰώδης "violet-coloured".

Iodine occurs in many oxidation states, including iodide (I−), iodate (IO−3), and the various periodate anions. It is the least abundant of the stable halogens, being the sixty-first most abundant element. It is the heaviest essential mineral nutrient. Iodine is essential in the synthesis of thyroid hormones. Iodine deficiency affects about two billion people and is the leading preventable cause of intellectual disabilities.

The dominant producers of iodine today are Chile and Japan. Iodine and its compounds are primarily used in nutrition. Due to its high atomic number and ease of attachment to organic compounds, it has also found favour as a non-toxic radiocontrast material. Because of the specificity of its uptake by the human body, radioactive isotopes of iodine can also be used to treat thyroid cancer. Iodine is also used as a catalyst in the industrial production of acetic acid and some polymers.


Iodine-123 (123I) is a radioactive isotope of iodine used in nuclear medicine imaging, including single photon emission computed tomography (SPECT) or SPECT/CT exams. The isotope's half-life is 13.22 hours; the decay by electron capture to tellurium-123 emits gamma radiation with a predominant energy of 159 keV (this is the gamma primarily used for imaging). In medical applications, the radiation is detected by a gamma camera. The isotope is typically applied as iodide-123, the anionic form.


Iodine-125 (125I) is a radioisotope of iodine which has uses in biological assays, nuclear medicine imaging and in radiation therapy as brachytherapy to treat a number of conditions, including prostate cancer, uveal melanomas, and brain tumors. It is the second longest-lived radioisotope of iodine, after iodine-129.

Its half-life is 59.49 days and it decays by electron capture to an excited state of tellurium-125. This state is not the metastable Te-125m, but rather a lower energy state that decays immediately by gamma decay with a maximum energy of 35 keV. Some of the excess energy of the excited Te-125 may be internally converted ejected electrons (also at 35 keV), or to x-rays (from electron bremsstrahlung), and also a total of 21 Auger electrons, which are produced at the low energies of 50 to 500 electron volts. Eventually, stable nonradioactive ground-state Te-125 is produced as the final decay product.

In medical applications, the internal conversion and Auger electrons cause little damage outside the cell which contains the isotope atom. The X-rays and gamma rays are of low enough energy to deliver a higher radiation dose selectively to nearby tissues, in "permanent" brachytherapy where the isotope capsules are left in place (I-125 competes with palladium-103 in such uses).Because of its relatively long half-life and emission of low-energy photons which can be detected by gamma-counter crystal detectors, I-125 is a preferred isotope for tagging antibodies in radioimmunoassay and other gamma-counting procedures involving proteins outside the body. The same properties of the isotope make it useful for brachytherapy (as noted), and for certain nuclear medicine scanning procedures, in which it is attached to proteins (albumin or fibrinogen), and where a longer half-life than provided by I-123 is required for test lasting several days.

Iodine-125 can be used in scanning/imaging the thyroid, but iodine-123 is preferred for this purpose, due to better radiation penetration and shorter half-life (13 hours). I-125 is useful for Glomerular filtration rate (GFR) testing in the diagnosis or monitoring of patients with kidney disease. Iodine-125 is used therapeutically in brachytherapy treatments of tumors. For radiotherapy ablation of tissues that absorb iodine (such as the thyroid), or that absorb an iodine-containing radiopharmaceutical, the beta-emitter iodine-131 is the preferred isotope.

125I is created by the electron capture decay of 125Xe, which is a synthetic isotope of xenon, itself created by neutron capture of stable 124Xe, which occurs naturally with an abundance of around 0.1%. Because of the synthetic production route of 125I and its short half-life, the natural abundance is effectively zero.


Iodine-129 (129I) is a long-lived radioisotope of iodine which occurs naturally, but also is of special interest in the monitoring and effects of man-made nuclear fission decay products, where it serves as both tracer and potential radiological contaminant.


Iodine-131 (131I) is an important radioisotope of iodine discovered by Glenn Seaborg and John Livingood in 1938 at the University of California, Berkeley. It has a radioactive decay half-life of about eight days. It is associated with nuclear energy, medical diagnostic and treatment procedures, and natural gas production. It also plays a major role as a radioactive isotope present in nuclear fission products, and was a significant contributor to the health hazards from open-air atomic bomb testing in the 1950s, and from the Chernobyl disaster, as well as being a large fraction of the contamination hazard in the first weeks in the Fukushima nuclear crisis. This is because I-131 is a major fission product of uranium and plutonium, comprising nearly 3% of the total products of fission (by weight). See fission product yield for a comparison with other radioactive fission products. I-131 is also a major fission product of uranium-233, produced from thorium.

Due to its mode of beta decay, iodine-131 is notable for causing mutation and death in cells that it penetrates, and other cells up to several millimeters away. For this reason, high doses of the isotope are sometimes less dangerous than low doses, since they tend to kill thyroid tissues that would otherwise become cancerous as a result of the radiation. For example, children treated with moderate dose of I-131 for thyroid adenomas had a detectable increase in thyroid cancer, but children treated with a much higher dose did not. Likewise, most studies of very-high-dose I-131 for treatment of Graves disease have failed to find any increase in thyroid cancer, even though there is linear increase in thyroid cancer risk with I-131 absorption at moderate doses. Thus, iodine-131 is increasingly less employed in small doses in medical use (especially in children), but increasingly is used only in large and maximal treatment doses, as a way of killing targeted tissues. This is known as "therapeutic use".

Iodine-131 can be "seen" by nuclear medicine imaging techniques (i.e., gamma cameras) whenever it is given for therapeutic use, since about 10% of its energy and radiation dose is via gamma radiation. However, since the other 90% of radiation (beta radiation) causes tissue damage without contributing to any ability to see or "image" the isotope, other less-damaging radioisotopes of iodine such as iodine-123 (see isotopes of iodine) are preferred in situations when only nuclear imaging is required. The isotope I-131 is still occasionally used for purely diagnostic (i.e., imaging) work, due to its low expense compared to other iodine radioisotopes. Very small medical imaging doses of I-131 have not shown any increase in thyroid cancer. The low-cost availability of I-131, in turn, is due to the relative ease of creating I-131 by neutron bombardment of natural tellurium in a nuclear reactor, then separating I-131 out by various simple methods (i.e., heating to drive off the volatile iodine). By contrast, other iodine radioisotopes are usually created by far more expensive techniques, starting with reactor radiation of expensive capsules of pressurized xenon gas.

Iodine-131 is also one of the most commonly used gamma-emitting radioactive industrial tracer. Radioactive tracer isotopes are injected with hydraulic fracturing fluid to determine the injection profile and location of fractures created by hydraulic fracturing.Much smaller incidental doses of iodine-131 than those used in medical therapeutic procedures, are supposed by some studies to be the major cause of increased thyroid cancers after accidental nuclear contamination. These studies suppose that cancers happen from residual tissue radiation damage caused by the I-131, and should appear mostly years after exposure, long after the I-131 has decayed. Other studies can't find a correlation.

Iodine (disambiguation)

Iodine is a chemical element with symbol I and atomic number 53.

Iodine may also refer to:

Isotopes of iodine:






Iodine clock reaction

Iodine (medical use)

Povidone-iodine, a common antiseptic

Tincture of iodine

Lugol's iodine

Iodine deficiency

Iodine Recordings

Iodine test

Iodine value

Little Iodine, a comics character

"Iodine", a song by Leonard Cohen from Death of a Ladies' Man

Iodine (film), a 2009 Canadian science-fiction film

"Iodine", a song by Icon For Hire from Scripted

Iofetamine (123I)

Iofetamine (iodine-123, 123I), brand names Perfusamine, SPECTamine), or N-isopropyl-(123I)-p-iodoamphetamine (IMP), is a lipid-soluble amine and radiopharmaceutical drug used in cerebral blood perfusion imaging with single photon emission computed tomography (SPECT). Labeled with the radioactive isotope iodine-123, it is approved for use in the United States as a diagnostic aid in determining the localization of and in the evaluation of non-lacunar stroke and complex partial seizures, as well as in the early diagnosis of Alzheimer's disease.An analogue of amphetamine, iofetamine has shown to inhibit the reuptake of serotonin and norepinephrine as well as induce the release of these neurotransmitters and of dopamine with similar potencies to other amphetamines like d-amphetamine and p-chloroamphetamine. In addition, on account of its high lipophilicity, iofetamine rapidly penetrates the blood-brain-barrier. Accordingly, though not known to have been reported in the medical literature, iofetamine likely possesses psychostimulant and possibly entactogenic effects. However, based on structure-activity relationships, it may also be highly neurotoxic to serotonergic and dopaminergic neurons similarly to most other para-halogenated amphetamines.

Isotopes of caesium

Caesium (55Cs) has 40 known isotopes, making it, along with barium and mercury, the element with the most isotopes. The atomic masses of these isotopes range from 112 to 151. Only one isotope, 133Cs, is stable. The longest-lived radioisotopes are 135Cs with a half-life of 2.3 million years, 137Cs with a half-life of 30.1671 years and 134Cs with a half-life of 2.0652 years. All other isotopes have half-lives less than 2 weeks, most under an hour.

Beginning in 1945 with the commencement of nuclear testing, caesium isotopes were released into the atmosphere where caesium is absorbed readily into solution and is returned to the surface of the earth as a component of radioactive fallout. Once caesium enters the ground water, it is deposited on soil surfaces and removed from the landscape primarily by particle transport. As a result, the input function of these isotopes can be estimated as a function of time.

Leo Szilard

Leo Szilard (; Hungarian: Szilárd Leó [ˈsilaːrd ˈlɛoː]; born Leó Spitz; February 11, 1898 – May 30, 1964) was a Hungarian-German-American physicist and inventor. He conceived the nuclear chain reaction in 1933, patented the idea of a nuclear reactor with Enrico Fermi in 1934, and in late 1939 wrote the letter for Albert Einstein's signature that resulted in the Manhattan Project that built the atomic bomb.

Szilard initially attended Palatine Joseph Technical University in Budapest, but his engineering studies were interrupted by service in the Austro-Hungarian Army during World War I. He left Hungary for Germany in 1919, enrolling at Technische Hochschule (Institute of Technology) in Berlin-Charlottenburg, but became bored with engineering and transferred to Friedrich Wilhelm University, where he studied physics. He wrote his doctoral thesis on Maxwell's demon, a long-standing puzzle in the philosophy of thermal and statistical physics. Szilard was the first to recognize the connection between thermodynamics and information theory.

In addition to the nuclear reactor, Szilard submitted patent applications for a linear accelerator in 1928, and a cyclotron in 1929. He also conceived the idea of an electron microscope. Between 1926 and 1930, he worked with Einstein on the development of the Einstein refrigerator. After Adolf Hitler became chancellor of Germany in 1933, Szilard urged his family and friends to flee Europe while they still could. He moved to England, where he helped found the Academic Assistance Council, an organization dedicated to helping refugee scholars find new jobs. While in England he discovered a means of isotope separation known as the Szilard–Chalmers effect.

Foreseeing another war in Europe, Szilard moved to the United States in 1938, where he worked with Enrico Fermi and Walter Zinn on means of creating a nuclear chain reaction. He was present when this was achieved on December 2, 1942. He worked for the Manhattan Project's Metallurgical Laboratory on aspects of nuclear reactor design. He drafted the Szilard petition advocating a demonstration of the atomic bomb, but the Interim Committee chose to use them against cities without warning.

After the war, Szilard switched to biology. He invented the chemostat, discovered feedback inhibition, and was involved in the first cloning of a human cell. He publicly sounded the alarm against the possible development of salted thermonuclear bombs, a new kind of nuclear weapon that might annihilate mankind. Diagnosed with bladder cancer in 1960, he underwent a cobalt-60 treatment that he had designed. He helped found the Salk Institute for Biological Studies, where he became a resident fellow. Szilard founded Council for a Livable World in 1962 to deliver "the sweet voice of reason" about nuclear weapons to Congress, the White House, and the American public. He died in his sleep of a heart attack in 1964. According to György Marx he was one of The Martians.

Nuclear fission product

Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release of heat energy (kinetic energy of the nuclei), and gamma rays. The two smaller nuclei are the fission products. (See also Fission products (by element)).

About 0.2% to 0.4% of fissions are ternary fissions, producing a third light nucleus such as helium-4 (90%) or tritium (7%).

The fission products themselves are usually unstable and therefore radioactive; due to being relatively neutron-rich for their atomic number, many of them quickly undergo beta decay. This releases additional energy in the form of beta particles, antineutrinos, and gamma rays. Thus, fission events normally result in beta and gamma radiation, even though this radiation is not produced directly by the fission event itself.

The produced radionuclides have varying half-lives, and therefore vary in radioactivity. For instance, strontium-89 and strontium-90 are produced in similar quantities in fission, and each nucleus decays by beta emission. But 90Sr has a 30-year half-life, and 89Sr a 50.5-day half-life. Thus in the 50.5 days it takes half the 89Sr atoms to decay, emitting the same number of beta particles as there were decays, less than 0.4% of the 90Sr atoms have decayed, emitting only 0.4% of the betas. The radioactive emission rate is highest for the shortest lived radionuclides, although they also decay the fastest. Additionally, less stable fission products are less likely to decay to stable nuclides, instead decaying to other radionuclides, which undergo further decay and radiation emission, adding to the radiation output. It is these short lived fission products that are the immediate hazard of spent fuel, and the energy output of the radiation also generates significant heat which must be considered when storing spent fuel. As there are hundreds of different radionuclides created, the initial radioactivity level fades quickly as short lived radionuclides decay, but never ceases completely as longer lived radionuclides make up more and more of the remaining unstable atoms.


(–)-2β-Carboisopropoxy-3β-(4-iodophenyl)tropane (RTI-4229-121, IPCIT) is a stimulant drug used in scientific research, which was developed in the early 1990s. RTI-121 is a phenyltropane based, highly selective dopamine reuptake inhibitor and is derived from methylecgonidine. RTI-121 is a potent and long-lasting stimulant, producing stimulant effects for more than 10 hours after a single dose in mice which would limit its potential uses in humans, as it might have significant abuse potential if used outside a medical setting. However RTI-121 occupies the dopamine transporter more slowly than cocaine, and so might have lower abuse potential than cocaine itself.


RTI(-4229)-55, also called RTI-55 or iometopane, is a phenyltropane-based psychostimulant used in scientific research and in some medical applications. This drug was first cited in 1991. RTI-55 is a non-selective dopamine reuptake inhibitor derived from methylecgonidine. However, more selective analogs are derived by conversion to "pyrrolidinoamido" RTI-229, for instance. Due to the large bulbous nature of the weakly electron withdrawing iodo halogen atom, RTI-55 is the most strongly serotonergic of the simple para-substituted troparil based analogs. In rodents RTI-55 actually caused death at a dosage of 100 mg/kg, whereas RTI-51 and RTI-31 did not. Another notable observation is the strong propensity of RTI-55 to cause locomotor activity enhancements, although in an earlier study, RTI-51 was actually even stronger than RTI-55 in shifting baseline LMA. This observation serves to highlight the disparities that can arise between studies.

RTI-55 is one of the most potent phenyltropane stimulants commercially available, which limits its use in humans, as it might have significant abuse potential if used outside a strictly controlled medical setting. However, it is definitely worthy of mentioning that increasing the size of the halogen atom attached to troparil serves to reduce the number of lever responses in a session when these analogs were compared in a study. Although RTI-55 wasn't specifically examined in this study the number of lever responses in a given session was of the order cocaine > WIN35428 > RTI-31 > RTI-51.

In contrast to RTI-31 which is predominantly dopaminergic, increasing the size of the covalently bonded halogen from a chlorine to an iodine markedly increases the affinity for the SERT, while retaining mostly all of its DAT blocking activity.

The radiopharmaceutical forms of RTI-55, in which the iodine atom is radioiodine so that the drug can be used in single-photon emission computed tomography, are called iometopane I 123 (USAN) or iometopane 123I (INN) and iometopane I 125 (USAN) or iometopane 125I (INN). The 123I and 125I isotopes are favored because they are very-high-energy γ-ray emitters.

Compared to the "WIN" compounds, extremely low Ki values are attainable.


A radioimmunoassay (RIA) is an immunoassay that uses radiolabeled molecules in a stepwise formation of immune complexes. A RIA is a very sensitive in vitro assay technique used to measure concentrations of substances, usually measuring antigen concentrations (for example, hormone levels in blood) by use of antibodies.

Although the RIA technique is extremely sensitive and extremely specific, requiring specialized equipment, it remains among the least expensive methods to perform such measurements. It requires special precautions and licensing, since radioactive substances are used.

In contrast, an immunoradiometric assay (IRMA) is an immunoassay that uses radiolabeled molecules but in an immediate rather than stepwise way.

A radioallergosorbent test (RAST) is an example of radioimmunoassay. It is used to detect the causative allergen for an allergy.

Sodium iodide

Sodium iodide (chemical formula NaI) is an ionic compound formed from the chemical reaction of sodium metal and iodine. Under standard conditions, it is a white, water-soluble solid comprising a 1:1 mix of sodium cations (Na+) and iodide anions (I−) in a crystal lattice. It is used mainly as a nutritional supplement and in organic chemistry. It is produced industrially as the salt formed when acidic iodides react with sodium hydroxide.


The thyroid gland, or simply the thyroid, is an endocrine gland in the neck, consisting of two lobes connected by an isthmus. It is found at the front of the neck, below the Adam's apple. The thyroid gland secretes three hormones, namely the two thyroid hormones (thyroxine/T4 and triiodothyronine/T3), and calcitonin. The thyroid hormones primarily influence the metabolic rate and protein synthesis, but they also have many other effects, including effects on development. Calcitonin plays a role in calcium homeostasis.Hormonal output from the thyroid is regulated by thyroid-stimulating hormone (TSH) secreted from the anterior pituitary gland, which itself is regulated by thyrotropin-releasing hormone (TRH) produced by the hypothalamus.The thyroid may be affected by several diseases. Hyperthyroidism occurs when the gland produces excessive amounts of thyroid hormones, the most common cause being Graves' disease, an autoimmune disorder. In contrast, hypothyroidism is a state of insufficient thyroid hormone production. Worldwide, the most common cause is iodine deficiency. Thyroid hormones are important for development, and hypothyroidism secondary to iodine deficiency remains the leading cause of preventable intellectual disability. In iodine-sufficient regions, the most common cause of hypothyroidism is Hashimoto's thyroiditis, also an autoimmune disorder. In addition, the thyroid gland may also develop several types of nodules and cancer.

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